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978-3-8439-1586-1, Reihe Physik
Nils Felix Kuhlmann
Lateral spin-valve devices operated by spin pumping
139 Seiten, Dissertation Universität Hamburg (2014), Softcover, A5
In this thesis, spin-valve devices that are operated by spin pumping are investigated. Spin valves are mesoscopic spin-electronic devices that consist of two ferromagnetic bars, called electrodes, and a nonmagnetic normal-metal strip that interconnects the electrodes. The devices are measured in nonlocal geometry. In contrast to conventional lateral spin valves, where a charge current generates a spin accumulation, in this thesis the spin-pumping effect is used as spin current injection mechanism. The first electrode is excited at ferromagnetic resonance and, therefore, due to spin pumping a nonequilibrium spin accumulation arises at the interface to the normal-metal strip. The resulting difference of the chemical potential between the normal-metal strip and the second electrode with static magnetization is measured as a voltage.
Prior to transport measurements, broadband-ferromagnetic resonance measurements are performed by means of vector-network analysis. With this technique, the resonance frequencies, coupling effects, and switching characteristics of the electrodes of the spin valves are characterized. The broadband-ferromagnetic resonance measurements enable to optimize the electrode design for spin-pumping experiments and to interpret transport measurements.
Transport experiments of spin-pumping operated spin valves are performed at room temperature. Nonlocal voltages in the range of nanovolts that depend on the magnetic excitation of the injector electrode are observed. The voltage signals are interpreted with a consistent model that introduces three voltage contributions: a baseline voltage, a heat correlated Seebeck voltage, and an asymmetric contribution that is related to the pumped spin current. The influence of the excitation frequency and the excitation power on the nonlocal voltage is analyzed. Advances of measurement technique and sample design are proposed.